Photographic films including photographic paper typically comprise a base material coated with a light-sensitive emulsion containing various silver salts and silver halides. During the developing process, the films are placed in solutions that chemically react with the emulsion layer in order to develop an image. During this process, some of the silver salts and silver halides dissolve and become suspended within the solutions. For example, one type of film developing solution, called a fixer, is specifically used to dissolve out any silver halides that were not exposed when the picture or image was taken.
Commercial film developing machines process such a large number of batches of photographs each day that recovery of the silver from the used developing fluids becomes cost-effective, even necessary. Of course, silver is a precious metal and is valuable even in fairly small quantities, which presents a financial incentive. Moreover, recovery of silver from commercial film developing operations is regulated by the government as an anti-pollution measure. Certain environmental regulations require that the amount of silver remaining in waste developing fluid be less than or equal to 0.3 parts per million (ppm).
Many commercial film developing machines are single units operated from small retail businesses, such as groceries, drugstores or stationers. Even these single retail units must comply with the relatively stringent environmental regulations and thus must implement a silver recovery process. Unfortunately, the equipment needed for silver recovery is often fairly expensive, and relatively complex to maintain. Understandably, the small business owner is reluctant to make a capital expenditure in such silver recovery equipment, and make the time and effort to learn how to operate it. Therefore, a significant portion of the silver recovery systems are leased and serviced under contract, which can be economically burdensome.
Of course, silver recovery is a relatively mature technology, as exhibited in U.S. Pat. No. 3,840,217 to MacKay, U.S. Pat. No. 4,441,697 to Peterson, et al., U.S. Pat. No. 5,004,212 to Gutierrez, and U.S. Pat. No. 5,472,176 to Azzara. The process typically involves passing the used film developing solution through a container in which is placed a screen, mesh, or porous filler of a metal that is above silver in the electromotive force series. For example, steel wool or woven screen wire may be used for the filler. The resulting galvanic replacement action causes silver to be deposited within the container as the metal is dissolved. After a predetermined time, contents of the container are sent to and further processed at a refinery to recover the silver.
Silver recovery systems available on the market all utilize a variable speed pump to insure an optimum flow rate through a container in which the galvanic reaction occurs. For example, a system denoted the METS model E-100 sold by CPAC equipment division of Leicester, N.Y., includes a variable speed pump that receives fluid directly from the film processing unit, or from an intermediate reservoir. The fluid is pumped to the lower portion of a first silver recovery container where it rises through the silver recovery filler to exit from an upper portion thereof. The precision metering pump is provided so that the fluid passes through the silver recovery container at the proper flow rate for optimum silver recovery.
Despite the availability of commercial silver recovery systems, there is a need for a simpler, more economical system.